This invention relates to novel chalking-resistant pigments, to their manufacture and to their use as opacifying extenders in exterior grade paint formulations. More specifically the invention relates to novel calcined clay pigments, the pigments being characterized by possessing a unique combination of coarse particle size and low oil absorption value.
The paint industry supplies consumer-oriented products of the solvent and emulsion types. Solvent paints are relatively simple systems, easy to formulate but difficult for the consumer to use. Solvent paints contain a binder (oil or resin), a solvent (thinner), drying agents and pigments. Emulsion or so-called "latex" paints are complex mixtures containing surfactants, protective colloids, biocides, freeze-thaw stabilizers, emulsifiers and water in addition to the one or more types of pigment which may be used. Following their introduction after World War II, latex paints have gained substantially in market acceptance. They now account for a majority of interior and exterior paint trade sales.
Interior and exterior latex paints have generally similar formulations. An important distinction however is that exterior grade paints contain relatively more binder and prime pigment but less extender pigment than interior paints. That is because paint film integrity and overall durability are more critical in exterior paints than in interior grades.
An important parameter in paint formulation is the pigment volume concentration, or PVC. PVC is a control factor in the design of paint formulations, because paint properties are governed by volume rather than weight effects. The following equation defines the PVC as a percentage of volume of dried paint film: ##EQU1##
The critical pigment volume concentration, or CPVC, is defined as that PVC at which air interfaces are generated in the dry paint film due to deficiency of binder with respect to pigment. It is well known that many paint volume properties change drastically at CPVC. The relationship between PVC and CPVC is nonlinear. There is authority for the view that different paints are properly compared on the basis of equal reduced pigment volume concentration, or RPVC. The RPVC is defined by the following: EQU RPVC=PVC/CPVC
Generally exterior grade paints have an RPVC less than 1 and interior paints have an RPVC greater than 1. There is considerable controversy over whether in latex paint the CPVC is a characteristic of the pigment or a characteristic of both pigment and binder. There is published authority for both points of view.
CPVC is related inversely to the amount of binder that the pigment particles "absorb". A conventionally used technique to determine this property of a pigment or extender is the amount of linseed oil needed to form a paste containing a given weight of pigment. This is referred to in the art as oil absorption. As used herein the term "oil absorption" refers to the procedure described in ASTM D-281. Substitution of an equal amount of high oil absorption extender pigment for one of low oil absorption results in a reduction of the CPVC of that paint. This in turn restricts the range of PVC that can be utilized in exterior formulations and the amount of extender pigments which can be employed.
Exterior grade latex paints contain a mixture of prime and extender pigments with titanium dioxide most generally used as the prime pigment because of its outstanding optical properties. Zinc oxide is employed to a smaller extent. The most commonly used extender pigments for exterior grade latex paints are calcium carbonate and talc. Kaolin clays are rarely used in exterior grade paint formulations, and when they are employed only small amounts are used for reasons which will be discussed below.
The binder in emulsion paints consists of globules (0.1 to 1.0 micron diameter) of film-forming polymer of 10,000 to 1,000,000 molecule weight. The latex particle size and composition are varied to effect changes in such properties as durability, gloss, glass transition temperature and the like. At present acrylic and vinyl-acrylic resins account for the majority of binders used in latex paints.
The weatherability of exterior grade coatings is determined by the ability of the coatings to resist chalking, fading and brittlement, gloss reduction, frosting and bleeding. Chalking, which is manifested by the formation of a powder on a painted surface, is one of the most undesirable performance characteristics of a paint. It involves the chemical degradation of the paint binder by atmospheric and meteorological attack, from which loose, removable powder (the pigment) is evolved from the paint film at or just beneath the surface. Two distinct mechanisms are believed to be responsible for chalking. One involves direct ultraviolet degradation of the binder. It is related to the ultraviolet stabilizer of the binder. With present-day use of ultraviolet screens in paints this is no longer much of a problem. The second mechanism occurs when a pigment acts as a catalyst for chemical oxidation of the binder. Thus it is evident that chalking is a characteristic of a paint film and the terms "chalking pigments" and "chalking-resistant pigments" as used herein will refer to chalking of the paint film containing pigment. Chalking is a problem still considered substantially unsolved by those in the paint industry, and methods for reducing chalking are continually sought by the paint industry and its suppliers.
Extender pigments profoundly influence the properties of latex paints. They control texture, optical and flow properties. Extenders involve a large group of materials with diverse chemical properties. In most published studies of extenders, the pigments have been evaluated in terms of PVC rather than RPVC. This makes quantitative comparisons of extenders difficult. Kaolin extender pigments are widely used in interior formulations, whereas the undesirable weathering properties of kaolin clay pigments have severely limited their use in exterior grade paint formulations. Several hypotheses have been suggested to describe chalking by TiO.sub.2, but little or no work has been done with extender pigments.
Commercially available, pigment-grade hydrous kaolins have oil absorption generally in the range 25-40 grams oil per 100 grams clay. Hydrous kaolins having the lower values in this range are desirable in exterior grade latex formulations but their chalking properties preclude such use. The high oil absorption values resulting from conventional calcination of kaolin clay pigments, which is typically in the range of 45-60 grams oil per 100 grams clay, preclude their use as prime extenders in latex paints formulated below CPVC.
An extensive study of simple extender pigment in vinyl polymer at 50% PVC is described by F. Liberti, Official Digest, vol. 33, March 1961, page 390. Liberti found that four particle sizes of talc, coarse (ASP.RTM. 400) and fine (ASP 100) hydrous kaolin, calcined kaolin (nonspecified particle sizes) and fine calcium carbonate all chalked at essentially the same rate, that is, these pigments all rated "fair" on a scale of good-fair-poor. Liberti also found that coarse calcium carbonate, rated as "good", chalked less than the fine calcium carbonate, but the RPVC was not controlled.
Kaolin clay pigments are supplied as pigments and extenders in uncalcined (hydrous) grades and calcined grades, the latter being favored where opacification (hiding power) is an important criterion. The hydrous grades include products composed predominantly of relatively fine and relatively coarse particles and are frequently supplied as blends of fractions of different particle sizes. Hydrous grades that contain an appreciable content of particles larger than 2 microns as determined by sedimentation include a significant quantity of naturally occurring stacks or booklets as well as the individual platelets known to characterize particles of kaolin that are finer than about 2 microns. The finer grades are composed predominantly of such individual platelets. Delaminated hydrous kaolins are produced by mechanically altering the naturally occurring stacks or booklets in whole (unfractionated) crude clays or coarse particle size fractions thereof. The delaminated grades generally have higher oil absorption values than naturally occurring clay of similar particle size distribution as determined by sedimentation. Most commercially available grades of calcined kaolin pigments have average particle sizes in the range of about 1.0 to 3 microns, and oil absorption values above 40 g./100 g. Generally oil absorption increases inversely with average particle size. Ultrafine grades of calcined clay having an average particle size below 1 micron are used as extenders and coating pigments by the paper industry. Oil absorption values exceed 80 g. oil/100 g. clay. Coarse particle size calcined clay pigments have been supplied by the industry as products having average particle sizes in the range of 4 to 7 microns with oil absorption values above 45 g./100 g. A process for preparing such products involving stage-wise fractionation of a dispersed pulp of crude clay to selectively reject undersized and oversized particles is described in U.S. Pat. No. 2,928,751 to Lyons. The calcined coarse kaolin pigment, as disclosed in this patent, is contemplated for use as an ingredient in the manufacture of ceramics.
U.S. Pat. No. 3,519,453 to Morris et al discloses blending calcined, delaminated clay with hydrous coating clays for coating applications in paper. Calcination is shown to increase oil absorption and coarsen particle size, mostly by agglomeration of fine material. U.S. Pat. No. 3,403,041 discloses the use of a chemically modified calcined mechanically delaminated clay in a paint formulated for interior use. U.S. Pat. No. 3,171,718 discloses (column 11) alkyd paint formulations containing calcined delaminated clay at 50% and 60% PVC. These paints are also formulated for interior use.
While hydrous and calcined clays have enjoyed widespread use as extenders for interior paints, to the best of my knowledge known forms of kaolin pigments or extenders, both hydrous and calcined grades, have not been used extensively by the paint industry as the principal extender in the formulation of exterior grade paints. Furthermore, to the best of my knowledge known forms of kaolin clay do not possess the combination of properties required for such use. These properties include adequate resistance to chalking, ability to be formulated at the high PVC necessary in exterior grade latex paint formulation and acceptable opacification properties.
It is known in the art that wet or dry milling of pulverized calcined clay pigments serve to breakup agglomerates in the calcined clay. This reduces oil absorption but has been associated with a significant loss of opacifying ability when the calcined clay pigments were evaluated in their intended end uses, namely in paper and in interior grade paints. Thus, post-milling has been a step avoided by suppliers of calcined clay pigments designed for prior uses of such clays .